The disclosure relates to an open loop and/or closed loop control method for an electropneumatic field device of a processing plant, such as a chemical plant, for example a petrochemical plant, a food-processing plant, for example a brewery, a power station or the like. An electropneumatic field devices can include a pneumatic actuating drive for operating a control member, such as a control valve, an electropneumatic converter for generating a pneumatic manipulating variable signal for the pneumatic actuating drive and open loop and/or closed loop control electronics with an electrical target value input for a command variable of a superordinate closed loop control system, for example from a control centre, for predetermining a target value, and an electrical output for a manipulating variable for piloting the electropneumatic converter. The pneumatic actuating drive can for example be connected to a valve member of a control valve, for example by an actuating rod for transmitting a translational actuating movement, or, if it is a rotary actuating drive, by an actuating shaft for transmitting a rotary actuating movement. An electropneumatic converter for generating a pneumatic control signal can for example provide an actuating pressure of 1 bar to 20 bar, for example, particularly 1 bar to 6 bar or 1 bar to 10 bar, for a pneumatic drive chamber of a pneumatic actuating drive. Pneumatic actuating drives can for example be equipped in a single-acting manner with spring return or a double-acting manner with two pneumatic working chambers acting counter to one another.
A method for operating a position regulator and a position regulator applying this method are described in DE 199 21 828 A1. For example, the position regulator has a first input for a command variable W for predetermining a target value, a second input for a controlled variable X and an output for a manipulating variable Y. The position regulator further comprises a closed loop controller, which generates a first output signal for forming the manipulating variable Y as a function of the input variable W and the controlled variable X. The position regulator has an error detector which detects whether the controlled variable X has assumed an incorrect operational state and which generates an error signal in the event of such an incorrect operational state. The error signal acts on a switch, which can deactivate the closed loop controller and activate an open loop controller. The open loop controller can be provided to provide a second output signal, assigned to an input variable W, for forming a manipulating variable Y. Thus, the open loop controller generates an output signal, which can be dependent on the input variable, for example linearly. The dependence of the output signal on a controlled variable can be determined during an initialization of the position regulator on the basis of the determination of a relationship between a manipulating variable and the input variable, which can be stored in a memory of the open loop controller.
DE 199 21 828 A1 describes that the controlled variable X is for example a position signal detected by a travel sensor of an actuating rod of a pneumatic actuating drive, which position signal is particularly prone to error as a consequence of the usual wear in typical travel sensors, which are realized as conductive plastics. In the event of an incorrect operational state of the measured position variable or controlled variable X, the known position regulator nevertheless ensures a still constantly reliable operating mode. The position regulator according to DE 199 21 828 A1 is suitable, in slowly changing processes and processes with only small disturbances, for setting the signal of the manipulating variable in such a manner that the actual valve position can always be kept close to its target value. This increases the operational reliability of a processing plant, which is equipped with such a position regulator, and avoids, as was previously necessary, having to immediately cause a position regulator to bring the control valve into a safe emergency off condition in the event of failure of the signal of the controlled variable. For moderately fast to fast changing processes, or processes with considerable disturbances, this position regulator is only suitable to a limited extent.
The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings.